Carrier for use in double-side polishing apparatus and method of double-side polishing wafer

ABSTRACT

A method provides a carrier used in a double-side polishing apparatus such that the carrier is disposed between upper and lower turn tables to which polishing pads are attached in the double-side polishing apparatus and holds a wafer interposed between the upper and lower turn tables in a holding hole formed in the carrier during polishing, including upper and lower main surface portions composed of a β-titanium alloy obtained from pure titanium containing 0.5 weight % or more of a β-stabilizing element. The carrier has a high abrasion resistance and can reduce its cost.

TECHNICAL FIELD

The present invention relates to a carrier configured to hold a wafer ina double-side polishing apparatus, and a method of double-side polishingthe wafer.

BACKGROUND ART

In conventional double-side polishing processes of wafers, the wafersare held with carriers for use in a double-side polishing apparatus, andthe carriers are disposed at a prescribed position between upper andlower turn tables of the double-side polishing apparatus. Each carrieris formed so as to have a thickness thinner than that of the wafer and aholding hole to hold the wafer. The wafer to be polished is inserted andheld in the holding hole. The upper and lower surfaces of the wafer areinterposed between polishing means such as polishing pads attached toopposing faces of the upper and lower turn tables and polished while apolishing agent is supplied to polishing surfaces.

In double-side polishing, each carrier is driven by a sun gear and aninternal gear, and the upper and lower main surfaces of this carrier,together with the surfaces of a wafer, are thereby polished. It is thusnecessary to use a carrier made of a material having a high strength toprevent the carrier from being damaged during double-side polishing.

The flatness of a wafer after double-side polishing depends on thedifference between the thickness of the carrier and the finishingthickness of the wafer. The carrier to be used accordingly has athickness such that this difference falls within a prescribed range,such as 0.5 μm or less.

Since the carrier is polished during wafer double-side polishing asabove, however, the thickness of the carrier decreases due to its wear.As the polishing operation is repeated, this decrease makes itimpossible to hold the difference between the carrier thickness and thewafer finishing thickness within the prescribed range while the waferfinishing thickness is maintained in a given range. This also makes itimpossible to achieve the required level of wafer flatness. When thefinishing thickness is managed to be in the range between 774 μm and 778μm, for example, such a difference cannot be maintained with a carrierworn by 4 μm. The carrier is disposed of when its thickness is decreasedto such an extent that the required level of wafer flatness cannot beachieved.

Conventional carriers are made of metal from the viewpoint of theirstrength. Carriers for use in polishing of silicon wafers are made oftitanium (Ti) because the elements belonging to groups 4A and 5A in theperiodic table are less likely to contaminate the silicon wafers (SeePatent Documents 1 and 2, for example).

CITATION LIST Patent Literature

Patent Document 1: Japanese Unexamined Patent publication (Kokai) No.2006-26760Patent Document 2: Japanese Unexamined Patent publication (Kokai) No.2008-23617

SUMMARY OF INVENTION Technical Problem

Use of a material having a high abrasion resistance for a carrier isneeded to increase its lifetime and decrease its cost. Titanium usuallyused for conventional carriers, however, is expensive and has a shortlifetime because of a low abrasion resistance, although titanium has ahigh strength. This leads to a cost problem of the carrier.

The present invention was accomplished in view of the above-describedproblems. It is an object of the present invention to provide a carrierfor use in a double-side polishing apparatus that has a high abrasionresistance and can reduce its cost.

Solution to Problem

To achieve this object, the present invention provides a carrier used ina double-side polishing apparatus such that the carrier is disposedbetween upper and lower turn tables to which polishing pads are attachedin the double-side polishing apparatus and holds a wafer interposedbetween the upper and lower turn tables in a holding hole formed in thecarrier during polishing, comprising upper and lower main surfaceportions composed of a β-titanium alloy obtained from pure titaniumcontaining 0.5 weight % or more of a β-stabilizing element.

Such a carrier has a high abrasion resistance, thereby enabling increasein its lifetime and reduction in its cost.

The entirety of the carrier may be composed of the β-titanium alloy.

Such a carrier has a high strength and can maintain its high abrasionresistance for a longer time for polishing.

Alternatively, the carrier may include a metal matrix and a coatingcomposed of the β-titanium alloy, the coating being formed so as tocover upper and lower main surfaces of the metal matrix.

Such a carrier can reduce its cost while achieving its high abrasionresistance due to the coating composed of the β-titanium alloy.

Moreover, the metal matrix is preferably composed of pure titanium orthe β-titanium alloy.

Such a carrier can eliminate the risk that when a silicon wafer, forexample, is polished, the carrier contaminates the silicon wafer. Inparticular, use of the metal matrix composed of pure titanium allows forreuse of carriers restored by coating, for example, existing worncarriers with the β-titanium alloy coating, thereby enabling furtherreduction in the cost.

Furthermore, the present invention provides a method of double-sidepolishing a wafer, comprising: disposing a carrier for use in adouble-side polishing apparatus according to the present inventionbetween upper and lower turn tables to which polishing pads areattached; and double-side polishing the wafer while holding the wafer inthe holding hole formed in the carrier.

Such a double-side polishing method can obtain wafers having therequired level of flatness by using the inventive carrier having a highabrasion resistance without changing the carrier over a long period oftime, thereby enabling reduction in the cost.

Advantageous Effects of Invention

The inventive carrier for use in a double-side polishing apparatusincludes upper and lower main surface portions composed of a β-titaniumalloy obtained from pure titanium containing 0.5 weight % or more of aβ-stabilizing element, and thereby has a high abrasion resistance and along lifetime. Use of this carrier for double-side polishing of a waferallows wafers having the required level of flatness to be obtainedwithout changing the carrier over a long period of time, therebyenabling great reduction in wafer production cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of an exemplary double-sidepolishing apparatus including a carrier according to the presentinvention;

FIG. 2 is a diagram of an internal structure of the double-sidepolishing apparatus shown in FIG. 1;

FIGS. 3A and 3B are schematic diagrams of examples of the inventivecarrier for use in a double-side polishing apparatus;

FIG. 4 is a graph of the result of the wear rate of carriers in exampleand comparative example; and

FIG. 5 is a graph of the result of GBIR of wafers in example andcomparative example.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be hereinafter described, butthe present invention is not limited to these embodiments.

A carrier for use in a double-side polishing apparatus according to thepresent invention will now be described.

The carrier for use in a double-side polishing apparatus (simplyreferred to as the carrier, below) according to the invention is used,for example, in the double-side polishing apparatus 20 as shown in FIGS.1 and 2. As shown in FIGS. 1 and 2, the double-side polishing apparatus20 includes an upper turn table 6 and a lower turn table 7 that aredisposed so as to face one another in the vertical direction. Polishingpads 8 are attached to the respective turn tables 6 and 7. At the centerbetween the upper turn table 6 and the lower turn table 7 is provided asun gear 9; at a circumferential portion therebetween is provided aninternal gear 10. As shown in FIG. 2, a holding hole 5 to hold a wafer Wis formed in the inventive carrier 1. During double-side polishing, thecarrier 1 is disposed between the upper turn table 6 and the lower turntable 7 with the wafer W held in the holding hole 5.

Some teeth of the sun gear 9 and the internal gear 10 engage withcorresponding teeth of the outer circumferential gear of the carrier.Rotating the upper turn table 6 and the lower turn table 7 by a drivingsource (not shown) causes the carrier 1 to rotate and revolve around thesun gear 9. During this operation, both the surfaces of the wafer W heldin the holding hole 5 of the carrier 1 are simultaneously polished bythe upper and lower polishing pads 8. During the polishing of the wafer,a polishing agent is supplied to the polished surfaces of the wafer froma nozzle (not shown).

In the polishing, since the upper and lower main surfaces of the carrier1 come in contact with the polishing pads 8, these main surfaces of thecarrier are polished together with the surfaces of the wafer andgradually wear out.

In view of this, as shown in FIG. 3A, an upper main surface portion 2and a lower main surface portion 3 of the inventive carrier 1 arecomposed of a β-titanium alloy obtained from pure titanium containing0.5 weight % or more of a β-stabilizing element. This type of carrierhas a high abrasion resistance, thereby allowing long-term maintenanceof the condition under which a wafer can be polished so as to have therequired level of flatness. In other words, the inventive carrier has alonger lifetime than does a conventional carrier composed of, forexample, pure titanium.

In the inventive carrier 1, the material of a portion other than themain surface portions is not particularly limited, provided the upperand lower main surface portions 2 and 3 are composed of the β-titaniumalloy, as above. As shown in FIG. 3B, for example, the carrier 1 can bethus constituted of a metal matrix 4 and a coating composed of theβ-titanium alloy formed so as to cover the upper and lower main surfacesof the metal matrix. This β-titanium alloy coating can be formed, forexample, by a sputtering method.

Such a carrier can achieve a high abrasion resistance by the upper andlower main surface portions 2 and 3 formed of the β-titanium alloycoating and reduce its cost by using a lower cost material for a portionother than the main surface portions. In addition, when repetitivepolishing of wafers causes the carrier to wear to the extent that therequired level of flatness cannot be achieved, the carrier can bethickened by forming the β-titanium alloy coating. This cansignificantly improve the frequency of reuse of the carrier, allowingthe cost to be significantly improved.

In this case, the metal matrix 4 may be composed of pure titanium. Inother words, an existing pure titanium carrier can be reused by beingcoated with the β-titanium alloy coating; this reuse enables furtherreduction in the cost. Alternatively, the metal matrix 4 may be composedof the β-titanium alloy so that the strength of the carrier isincreased. Both cases are preferable because the risk that a siliconwafer is contaminated during polishing of the silicon wafer can beeliminated.

The β-stabilizing element is not particularly limited; examples thereofinclude V, Zr, Nb, Mo, Hf, Cr, Mn, Fe, Co, and Ni. Considering Fe is aninexpensive metal that is not a rare metal and has a low diffusioncoefficient for silicon wafers, a preferable β-stabilizing element isFe.

The content of the β-stabilizing element is equal to or more than 0.5weight %. In particular, this content is preferably 1.5 weight % or morefrom the viewpoint of the abrasion resistance; this content is alsopreferably 2.0 weight % or less from the viewpoint of the inhibition ofthe contamination of silicon wafers. The content however is notparticularly limited, provided it is 0.5 weight % or more.

Each of the carriers 1 shown in FIG. 2 is configured to have a singleholding hole 5 to hold one wafer W. The invention is not limited to thisconfiguration. For example, the carrier may have plural holding holes 5so that the carrier holds plural wafers W.

Moreover, a resin insert to protect the edge of a wafer from beingdamaged by the carrier may be attached along the inner circumference ofthe holding hole 5.

A method of double-side polishing wafers according to the invention willnow be described.

This embodiment of the inventive method of double-side polishing wafersuses the inventive carriers described above.

As shown in FIG. 1, first, the inventive carriers 1 including at leastthe main surface portions composed of the β-titanium alloy are disposedbetween the polishing pads 8 attached to the upper and lower turn tables6 and 7 of the double-side polishing apparatus 20.

Next, wafers W are inserted and held in the holding hole 5 of therespective disposed carriers 1.

Next, the upper and lower surfaces of the wafers W are interposedbetween the polishing pads 8 attached to the upper and lower turn tables6 and 7. While a polishing agent is supplied to the polishing surfaces,both the surfaces of the wafers are polished. Polishing conditions inthis polishing process may be the same as conventionally.

The double-side polishing thus performed can obtain wafers having therequired level of flatness by using the inventive carriers having a highabrasion resistance without changing the carriers over a long period oftime, thereby reducing the cost.

Example

The present invention will be more specifically described below withreference to an example and a comparative example, but the presentinvention is not limited to this example.

Example

A double-side polishing apparatus, as shown in FIG. 1, having theinventive carriers shown in FIG. 3A was used to evaluate the wear rateof the carriers.

Each of the inventive carriers was produced such that the entire carrierwas composed of a β-titanium alloy obtained from pure titaniumcontaining Fe. In this production, the Fe content of the β-titaniumalloy was changed to obtain five types of carriers: carrier A (0.5weight %), carrier B (1.0 weight %), carrier C (1.5 weight %), andcarrier D (2.0 weight %); five carriers were produced every carriertype. The thickness of these carriers was 770 μm. A resin insert wasattached along the inner circumference of each holding hole. Note thatthe Fe content was measured by X-ray fluorescence analysis.

The wear rate of the carriers was evaluated in a manner that, as shownin FIG. 2, the same type of five carriers having the same Fe contentholding no wafer were disposed in the double-side polishing apparatus,and the double-side polishing apparatus was operated in the same manneras in polishing of wafers to calculate the wear rate of the carriersfrom a decrease in carrier thickness per hour.

The polishing conditions were as follows:

Urethane foam polishing pads were used;

An alkaline solution containing colloidal silica was used as a polishingagent and the solution was recycled; and

The pressure applied to the surfaces of the carriers was 200 g/cm².

The thickness of the carriers was measured at 400 points along theentire circumference of the holding hole. The average value of themeasured thickness was used to calculate the wear rate.

The result of the wear rate is given in Table 1 and FIG. 4. As shown inTable 1 and FIG. 4, the wear rate was greatly decreased compared withthe later-described comparative example. The wear rate of the carriersdecreased with an increase in the Fe content of the β-titanium alloy.The decrease in the wear rate, however, substantially terminated at a Fecontent of 1.5 weight % or more (carrier C and carrier D). The wear rateafter the termination was 0.04 μm/h.

Next, 300-mm-diameter silicon wafers were double-side polished with thesame carriers as the carriers D (having a Fe content of 2.0 weight %)except that the thickness of the carriers was 771 μm. The flatness,Global Back-Side Ideal Range (GBIR), of the polished wafers wasevaluated. In the polishing, the target finishing thickness of thewafers was 775 μm.

The polishing conditions were as follows: the same double-side polishingapparatus and polishing agent as in the evaluation of the wear rate ofthe carriers were used to polish five wafers per one batch. The pressureapplied to the surfaces of the wafers to be polished was set at 200g/cm².

The GBIR of the polished wafers was measured with a flatness measuringinstrument (Nanometoro 300TT made by KURODA Precision Industries Inc.,).The GBIR of the wafers was obtained by calculating an average of GBIRvalues of the five polished wafers in the same batch.

The result of the GBIR is given in FIG. 5. As shown in FIG. 5, theexample demonstrated that the GBIR began to decrease when the usage timeof the carriers exceeded 45,000 minutes, and the GBIR was 0.25 μm whenthe usage time of the carriers was 50,000 minutes. At this time, thecarrier thickness was 767 μm.

The later-described comparative example, on the other hand, demonstratedthat the wafer flatness was 0.35 μm when the usage time of the carrierswas 20,000 minutes.

Thus, this example inhibited the degradation of the wafer flatness for alonger period of time and exhibited a longer carrier lifetime comparedwith the later-described comparative example.

The main surfaces of the carriers used for 50,000 minutes in the examplewere then coated with a β-titanium alloy having a Fe content of 2.0weight % by the Ar sputtering method. The thickness of this β-titaniumalloy coating was 4 μm in a total of the upper and lower main surfaces.The thickness of the coated carriers was 771 μm, which was the same asthat before the start of polishing.

With these coated carriers, double-side polishing of wafers wasperformed under the same conditions. The result was that the GBIR of thepolished wafers was 0.17 μm; thus the degradation of the GBIR wassettled.

The wafer flatness was not degraded even when the carrier was then usedfor 15,000 minutes (65,000 minutes passed after the start of the usage).

Comparative Example

The carrier wear rate and the wafer flatness (GBIR) were evaluated underthe same conditions as in the example except that a carrier composed ofa β-titanium alloy obtained from pure titanium containing 0.2 weight %of Fe.

The result of the wear rate is given in Table 1 and FIG. 4. As shown inTable 1 and FIG. 4, the wear rate was 0.13 μm/h and became worse thanthat in the example. The wear rate of a conventional pure titaniumcarrier (containing no β-stabilizing element) is usually 0.14 μm/h.Compared to this pure titanium carrier, the comparative example merelyexerted a small effect in improving the wear rate.

The result of the GBIR is given in FIG. 5. As shown in FIG. 5, the GBIRbegan to decrease when the usage time of the carriers exceeded 12,500minutes, and the wafer flatness was 0.35 μm when the usage time of thecarrier was 20,000 minutes. At this time, the carrier thickness was 765μm. Thus, the comparative example degraded the flatness in a shortertime and exhibited a shorter carrier lifetime compared with the example.

TABLE 1 COMPARATIVE EXAMPLE EXAMPLE Fe CONTENT IN Ti 2.0 1.5 1.0 0.5 0.2(wt %) WEAR RATE (μm/h) 0.04 0.04 0.05 0.06 0.13

It is to be noted that the present invention is not limited to theforegoing embodiment. The embodiment is just an exemplification, and anyexamples that have substantially the same feature and demonstrate thesame functions and effects as those in the technical concept describedin claims of the present invention are included in the technical scopeof the present invention.

1. A carrier used in a double-side polishing apparatus such that thecarrier is disposed between upper and lower turn tables to whichpolishing pads are attached in the double-side polishing apparatus andholds a wafer interposed between the upper and lower turn tables in aholding hole formed in the carrier during polishing, comprising upperand lower main surface portions composed of a β-titanium alloy obtainedfrom pure titanium containing 0.5 weight % or more of a β-stabilizingelement.
 2. The carrier according to claim 1, wherein the entirety ofthe carrier is composed of the β-titanium alloy.
 3. The carrieraccording to claim 1, comprising a metal matrix and a coating composedof the β-titanium alloy, the coating being formed so as to cover upperand lower main surfaces of the metal matrix.
 4. The carrier according toclaim 3, wherein the metal matrix is composed of pure titanium or theβ-titanium alloy.
 5. A method of double-side polishing a wafer,comprising: disposing a carrier for use in a double-side polishingapparatus according to claim 1 between upper and lower turn tables towhich polishing pads are attached; and double-side polishing the waferwhile holding the wafer in the holding hole formed in the carrier.
 6. Amethod of double-side polishing a wafer, comprising: disposing a carrierfor use in a double-side polishing apparatus according to claim 2between upper and lower turn tables to which polishing pads areattached; and double-side polishing the wafer while holding the wafer inthe holding hole formed in the carrier.
 7. A method of double-sidepolishing a wafer, comprising: disposing a carrier for use in adouble-side polishing apparatus according to claim 3 between upper andlower turn tables to which polishing pads are attached; and double-sidepolishing the wafer while holding the wafer in the holding hole formedin the carrier.
 8. A method of double-side polishing a wafer,comprising: disposing a carrier for use in a double-side polishingapparatus according to claim 4 between upper and lower turn tables towhich polishing pads are attached; and double-side polishing the waferwhile holding the wafer in the holding hole formed in the carrier.